This is a "distance" type problem, and you can use the formula d=r*t to solve it, where d is distance, r is rate of speed, and t is time. I am going to use subscripts on the variables to tell them apart, p for private jet and c for commercial jet.
For the private jet, all we know is that the flight took 6 hours, so d=r*6 which is dp=6rp.
For the commercial jet, we know the time is 2.5 hours and we know its rate of speed compared to the private jet, so the equation for the commerical jet is dc=(3rp-75)* 2.5
Notice that the rate in this equation is the rate of the private jet, rp
The final piece of the puzzle is that the distance is the same for both jets, dp=dc so we can set the two quantities equal to each other:
6rp = (3rp-75)* 2.5
Since we have the same variable in both places in the equation, we can leave the subscripts off now to make the equation simpler.
6r = (3r - 75) * 2.5
6r = 2.5(3r - 75)
6r = 7.5r - 187.5
-7.5 r -7.5r
-1.5r = -187.5
r = 125
r was the rate of speed for the private jet; Evaluate 3(125) - 75 to get the speed of the commercial jet.
Hope this helps!